Deep-water Renewal Research Articles

The sources of Antarctic bottom water in a global ice–ocean model

Two mechanisms contribute to the formation of Antarctic bottom water (AABW). The first, and probably the most important, is initiated by the brine released on the Antarctic continental shelf during ice formation which is responsible for an increase in salinity. After mixing with ambient water at the shelf break, this salty and dense water sinks along the shelf slope and invades the deepest part of the global ocean. For the second one, the increase of surface water density is due to strong cooling at the ocean–atmosphere interface, together with a contribution from brine release. This induces deep convection and the renewal of deep waters. The relative importance of these two mechanisms is investigated in a global coupled ice–ocean model. Chlorofluorocarbon (CFC) concentrations simulated by the model compare favourably with observations, suggesting a reasonable deep water ventilation in the Southern Ocean, except close to Antarctica where concentrations are too high. Two artificial passive tracers released at surface on the Antarctic continental shelf and in the open-ocean allow to show clearly that the two mechanisms contribute significantly to the renewal of AABW in the model. This indicates that open-ocean convection is overestimated in our simulation. Additional experiments show that the amount of AABW production due to the export of dense shelf waters is quite sensitive to the parameterisation of the effect of downsloping and meso-scale eddies. Nevertheless, shelf waters always contribute significantly to deep water renewal. Besides, increasing the P.R. Gent, J.C. McWilliams [Journal of Physical Oceanography 20 (1990) 150–155] thickness diffusion can nearly suppress the AABW formation by open-ocean convection.

Open-ocean convection and polynya formation in a large-scale ice–ocean model

The physical processes responsible for the formation in a large-scale ice–ocean model of anoffshore polynya near the Greenwich meridian in the Southern Ocean are analysed. In thisarea, the brine release during ice formation in autumn is sufficient to destabilise the watercolumn and trigger convection. This incorporates relatively warm water into the surface layerwhich, in a first step, slows down ice formation. In a second step, it gives rise to ice meltinguntil the total disappearance of the ice at the end of September. Two elements are crucial forthe polynya opening. The first one is a strong ice-transport divergence in fall induced by southeasterlywinds, which enhances the amount of local ice formation and thus of brine release. Thesecond is an inflow of relatively warm water at depth originating from the Antarctic CircumpolarCurrent, that sustains the intense vertical heat flux in the ocean during convection. The simulatedpolynya occurs in a region where such features have been frequently observed. Nevertheless, the model polynya is too wide and persistent. In addition, it develops each year, contrary toobservations. The use of a climatological forcing with no interannual variability is the majorcause of these deficiencies, the simulated too low density in the deep Southern Ocean and thecoarse resolution of the model playing also a role. A passive tracer released in the polynya areaindicates that the water mass produced there contributes significantly to the renewal of deepwater in the Weddell Gyre and that it is a major component of the Antarctic Bottom Water(AABW) inflow into the model Atlantic.

A Numerical Study of the Dynamics of the Riverine Thermal Bar in a Deep Lake

A numerical model based on a Finite Volume formulation of the Navier–Stokes equations is used to simulate a range of scenarios leading to a thermal bar formed by a river inflow to an idealised deep lake. The results presented here show that small riverine salinity increases have a profound effect on the dynamics of the thermal bar, suppressing horizontal propagation of the plume and raising the possibility of a thermal bar which is capable of sinking to great depths. This finding is particularly relevant to Lake Baikal in Siberia, where the vigorous deep-water renewal is still not fully understood. An analysis of the buoyancy forces governing the depth of penetration of the thermal bar plume shows that realistic salinity gradients are an important factor in determining the circulation of Baikal waters. Observations of the saline curtailment of the thermal bar's horizontal propagation also reveal a potential for reduced productivity in the ecosystem of any temperate river delta during the Spring renewal period.

Primary production and deep-water oxygen content of two British Columbian fjords

Abstract Vertical profiles of 14C-uptake were acquired monthly from the mouths and landward stations of periodically anoxic Saanich Inlet and oxygenated Jervis Inlet, British Columbia, Canada from August 1985 to October 1989. Saanich Inlet (490 g C m−2 year−1) was 1.7 times more productive than Jervis Inlet (290 g C m−2 year−1) and primary production toward the mouths of both inlets was 1.4 times higher than at the landward stations. The elevated rates of primary production in Saanich Inlet may have been due to exchange with the nutrient-rich surface waters of the passages leading to the Pacific Ocean and the up-inlet gradients in both fjords also may have reflected relative nutrient supply. Sediment-trap results show enhanced fluxes of biogenic silica to the deep waters of Saanich Inlet; associated organic matter is likely to have caused a large oxygen demand. Combined with the high primary production and export flux, low rates of vertical mixing and particle-entrapment within the fjord, factors associated with weak estuarine circulation as well as weak winds and tides in Saanich Inlet, may also stimulate anoxia. Although in Jervis Inlet there is more stagnant water behind the sill and deep-water renewals appear to be less frequent than in Saanich Inlet, the deep sill allows degradation of a significant fraction of the sinking organic matter before the stagnant waters are reached, reducing the chances of oxygen depletion in the bottom waters.

Living (Stained) Benthic Foraminifera and their Response to the Seasonal Hydrographic Cycle, Periodic Hypoxia and to Primary Production in Havstens Fjord on the Swedish West Coast

Havstens Fjord on the Swedish west coast is a silled fjord that is characterized by strong stratification and stagnant bottom-water with periodically occurring hypoxic (0–2mll−1) or anoxic (≤0mll−1) conditions. Renewal of Havstens Fjord's deep-water occurs every year, mostly during winter or spring. The aim of this study was to discover how living benthic Foraminifera respond to hydrographic variations, periodic oxygen deficiency and variations in primary production. A long series of monthly hydrographic measurements combined with sediment sampling were performed from August 1993 to December 1994. Sampling was carried out at four different sites along a depth transect (12, 20, 30 and 40m). Monthly values of chlorophyll a from the water surface down to 15m were obtained. With increasing water depth, the foraminiferal fauna changed from a low diverse brackish fauna, through a diverse and abundant fauna, to a low diversity and sparse fauna characterized by species tolerant of oxygen depletion. At the deepest site (40m) Elphidium magellanicum and Stainforthia fusiformis survived five months of anoxic or near anoxic conditions. At 30m the periodic hypoxic conditions were severe enough to prevent a rich benthic macrofauna establishing, but there was enough oxygen for an abundant and diverse foraminiferal fauna to thrive. Under oxic conditions, freshly sedimented phytoplankton seem to be an important food source for the Foraminifera.

Laboratory and numerical simulation of the Skagerrak circulation

A rotating laboratory model and two, three-dimensional numerical hydrodynamic models are used to simulate the stationary circulation of the Skagerrak region. The laboratory experiment is used as a reference solution (benchmark) for numerical simulations with the same set-up and forcing, except for some minor details. The laboratory model is validated to field data for 3 different processes: the Atlantic Water circulation along the bottom slopes, the Skagen front at the mouth of the Kattegat and deep-water renewal in Oslofjorden. The benchmark includes a map of the surface currents, several current velocity sections, salinity distributions and the response of the water column outside Oslofjorden to the spreading and dilution of tracers discharged in the German Bight and the Kattegat. Wind forcing and time-dependent flows like tides and frontal instabilities are not included in the benchmark. Local wind forcing is not simulated; however, the advective boundary conditions are a result of remote, regional meteorological forcing. The main validation to field data is for the SKAGEX experiment, during which time the local wind forcing was weak. Two numerical models are used in this study. A non-hydrostatic model MIKE 3 that employs an artificial compressibility technique and a hydrostatic model, the Princeton Ocean Model (POM), which uses sigma coordinates in the vertical. The results show that both models are capable of simulating large-scale geophysical flows in a region with steep topography and large density gradients. In the former case, improved results are obtained when the artificial compressibility is chosen as a calibration parameter. In the latter case a z-level interpolation method is used to help minimize potential pressure gradient errors and reasonable results are obtained with this model. However, the different response of the two numerical models highlights the need for model improvements in order to enable the simulation of the strong horizontal current shears in these waters. The results show the importance of potential vorticity dynamics in steering the slope currents and the baroclinic jets along the coasts.

On the vertical distribution of microorganisms in lake baikal during spring deep-water renewal

The vertical distribution of microorganisms during spring deep-water renewal in Lake Baikal was studied. The downward advection of trophogenic waters was found to create conditions for the extensive growth of microorganisms capable of decomposing and mineralizing organic carbon, nitrogen, and phosphorus in deep water layers. These processes occur annually at spring thermal bars near the underwater slope of Lake Baikal, whereas in its pelagic zone, the deep intrusions of waters rich in organic material are observed only in the years when enhanced deep-water renewal is accompanied by a high spring yield of phytoplankton.

Time-Dependent Modeling of the Baltic Entrance Area. 2. Water and Salt Exchange of the Baltic Sea

A time-dependent model for stratification and circulation within the Baltic entrance area (Gustafsson 2000) is tested against observed salinities for the period 1961–1993. Although the Baltic Sea is one of the largest estuarine systems on earth, this model could be applicable to smaller estuarine systems and embayments with tidal exchange. The seasonal cycle of freshwater flux across the sill area does not follow the seasonal cycle of freshwater supply to the Baltic Sea. The seasonal variation of the flux is a combined effect of the seasonal variation in freshwater supply, in Baltic mean sea level, and in dispersion of salt across the sills. The seasonal variation in dispersion of salt is due to the seasonal cycle of sea level variability. The model is used to predict the inflow of high saline water to the Baltic Sea. The resulting inflow time-series is consistent with variations in the deep-water salinity and temperature in the deeper parts of the Baltic Sea. A comparison with previous estimates of the magnitude of major Baltic inflows shows that the model is able to reproduce the characteristics fairly well although the magnitude of the flows of water and salt appears lower than other estimates. It is shown that a climatic change that increases the wind mixing does not significantly change the major inflows. Both increased amplitudes of sea level variations in the Kattegat and decreased freshwater supply to the Baltic Sea substantially increase the magnitude of the inflows. It is shown that deep-water renewal in the Baltic Sea is obstructed during years with high freshwater supply even if the sea level forcing is favorable to a major inflow.

Toward an Understanding of Deep-Water Renewal in the Eastern Mediterranean

Abstract This paper presents a scenario for understanding the unexpectedly large changes of deep waters and thermohaline circulation in the Eastern Mediterranean during the past decade. It is demonstrated as a possible deep-water renewal mechanism in the Eastern Mediterranean in a numerical simulation with a high-resolution model, which has successfully reproduced the observed 1987 and 1995 regimes as shown by the two Meteor cruises. A budget study of the model simulation has shown that more than 75% of the salt added to the deep layers of the Eastern Mediterranean could have come from the top 1000 m by a salinity redistribution process triggered by intensive cooling over the Aegean Sea. A water transformation process analysis is carried out in the model simulation to reveal how colder and fresher dense deep water formed in the Aegean is turned into the Eastern Mediterranean Deep Water (EMDW) as observed. Surface heat and freshwater fluxes are diagnosed to show the roles of each component during the trans...

Analysis of deep-water exchange in the Caspian Sea based on environmental tracers

In order to quantify deep-water exchange in the Caspian Sea, the world's largest inland water body, water samples were analyzed for the transient tracers H 3 , He 3 , He 4 , CFC-11, CFC-12 and atmospheric noble gases. Measurements of temperature, salinity (calculated from conductivity for the ionic composition of Caspian Sea water), and dissolved oxygen were employed to investigate the processes responsible for deep-water renewal. The Caspian Sea consists of two deep basins, the southern and central basins, separated by a sill, and a shallow northern basin. The deep water (below 200 m) accounts for almost 60% of the total water mass. Below 200 m the concentrations of H 3 and He 3 are much lower in the southern basin than at the same depths in the central basin, but this is not the case for either of the CFCs. However, apparent water ages calculated from H 3 – He 3 and from CFC-12 concentrations are the same for the deep water of the southern and central basins, and yield deep-water exchange rates of approximately 7% per year for each of the two basins. This implies volume fluxes across the 200-m level of about 2220 km 3 yr −1 within the southern basin and 770 km 3 yr −1 within the central basin. Based on the apparent water ages, the oxygen depletion in the deep water is estimated to be about 0.35 mg l −1 yr −1. The processes responsible for deep-water exchange have not yet been identified conclusively. However, vertical temperature and salinity gradients observed during two expeditions, in September 1995 and 1996, suggest that within the southern and central basins large-scale convection cannot be triggered by seasonal cooling alone, but requires the surface water to be cold/saline or to contain high suspended sediment loads. In the central basin the increase in salinity occurring during ice formation in early winter is possibly sufficient to cause convection. In late summer, the horizontal transport of water from the upper 170 m of the central basin into the southern basin results in mixing down to 400 m. In winter this process might result in convection down to the maximum depth. Alternatively, the data are also consistent with the hypothesis that rare events cause large-scale convection down to the maximum depth in the southern and in the central basin simultaneously, followed by slight mixing that mainly affects only the top 500 m. According to apparent water ages from below 500 m, the last such major mixing event could have occurred in 1976 shortly before the water level of the Caspian Sea began to rise.

Is Bottom Boundary Layer Mixing Slowly Ventilating Greenland Sea Deep Water*

Abstract Bottom water temperatures in the central Greenland Sea have been increasing for the last two decades. The warming is most likely related to the absence of deep convective mixing, which cools and freshens the deep water. However, recent observations confirm a slow and steady increase of anthropogenic tracers such as chlorofluorocarbons (CFCs). This points to some amount of bottom water “ventilation” in the absence of deep convective mixing and poses a challenge to our understanding of deep water renewal. One explanation for the observed trends in both temperature and CFCs is significant vertical mixing. The basin-averaged diapycnal diffusivity, required to explain both trends, kυ,av ∼ 2–3 (×10−3 m2 s−1), is very unlikely to occur in the interior of the ocean. However, a diffusivity of kυ,bbl ∼ 10−2 m2 s−1 within a 150-m thick bottom boundary layer would be sufficient to explain the deep tracer increase. The implications of a secondary circulation driven by such large boundary layer mixing are disc...

Sensitivity of Ventilation Rates and Radiocarbon Uptake to Subgrid-Scale Mixing in Ocean Models

The sensitivity of ventilation timescales and radiocarbon (14C) uptake to subgrid-scale mixing parameterization is studied in a global ocean model. Seven experiments are examined that are identical in every manner except their representation of subgrid-scale mixing of tracers. The cases include (i) two runs with traditional Cartesian mixing (HOR), (ii) a run with enhanced isopycnal mixing (ISO), and (iii) four runs in which the effects of eddies on the mean ocean flow are parameterized following Gent and McWilliams (GM). Horizontal, isopycnal, and isopycnal-thickness diffusion coefficients are varied sequentially in the model runs. Of particular interest is the role of the tracer mixing schemes in influencing longer timescale ventilation processes—centennial and beyond—such as deep water mass renewal and circulation. Simulated ventilation timescales and 14C vary greatly between the three mixing schemes. The isopycnal mixing run exhibits the most rapid water mass renewal due to strong diffusion effects and excessive surface convective overturn, particularly in the Southern Ocean. In contrast, the GM cases show much more gradual renewal of deep and bottom waters, with limited vertical convection of surface waters and slower abyssal currents. Under GM, a background horizontal diffusion or altered isopycnal mixing do not significantly change interior ocean ventilation rates. This means modelers can adjust these background diffusion coefficients under GM (for numerical purposes) without significantly changing model ventilation rates. Reducing the GM isopycnal thickness diffusivity, on the other hand, noticeably increases simulated deep water ventilation rates. In comparison with the HOR runs, deep and bottom water ventilation timescales are reduced by about 30% in ISO, and increased by 30%–40% under GM. Comparison is made between model simulated and observed 14C. The GM runs appear to be the least successful in the North Atlantic Ocean, exhibiting very gradual and only shallow water-mass renewal compared to observations. In the Pacific and Indian Oceans, the HOR and ISO runs are ventilated too rapidly due to strong convection and water-mass contribution from the Southern Ocean. In contrast, the GM runs simulate spuriously old and 14C-depleted bottom and middepth water. The GM cases do, however, capture realistic 14C in the upper 1500 m of the Indian and Pacific Oceans. Overall, none of the model cases reproduce global ocean ventilation rates over centennial timescales (under the chosen set of parameter values). Higher horizontal resolution and a spatially varying GM thickness diffusivity may be required before global models capture long timescale ocean renewal processes with some degree of fidelity.

Modeling of deep-water renewal through cold convective plumes

In only a few regions in the world ocean, surface water is transformed into deep water by open ocean convection. In this paper, small areas of vertical flow, or plumes, are studied using a steady-state model, integrated in streamline coordinates. We use an entrainment hypothesis based on knowledge about in-mixing of water across the shear zone from an area of low turbulence into an area of high turbulence. This results in a model using conservation equations for momentum, heat and salt, and a nonlinear equation of state. Effects of earth rotation are included. The depth reached by surface water when moving through stationary plumes is predicted, and found strongly dependent on the properties in the surface layer. This layer has to be colder but only slightly denser than the water just below it to be a source for steady-state plumes. The thermobaric effect, the dependency of compressibility on the temperature, plays a major role for the plume penetration depth. In weakly stratified cold water, like in the Greenland Sea, deep convection is found to be possible and efficient through steady-state plumes. Initial excess density of less than 5×10 −3 kg m −3 , caused by low temperature, can give convection deeper than 1500 m through plumes with horizontal scales of a few hundred meters and vertical velocities of order a few tens of centimeters per second.

The Odden ice tongue and Greenland Sea convection

The Greenland Sea is one of only four areas in the world ocean where ventilation occurs through open ocean convective renewal of intermediate and deep waters in winter, the others being the Labrador Sea, the western Mediterranean and the Antarctic. These regions occupy only about one thousandth of the area of the world's oceans, yet they are of global importance since it is only by this means that a complete vertical as well as horizontal circulation of the ocean can take place, with dissolved gases and nutrients cycling back into the depths. The whole world-wide circulating system is called the thermohaline circulation or, colloquially, the Great Ocean Conveyor Belt (Broecker 1995). Any change in the driving of this conveyor belt through a variation in the rate of convection would lead to world-wide effects on climate, and it has been suggested that changes of this nature may have been responsible for some of the rapid climatic fluctuations detected in Greenland ice cores. In the Greenland Sea, convection occurs in the centre of a cyclonic gyre which is bounded: (i) to the west by a cold current (the East Greenland Current, EGC) advecting ice and water of polar origin into the system from the Arctic Basin, (ii) to the east by a warm northward-flowing current (the West Spitsbergen Current), and (iii) to the south by the Jan Mayen Current (JMC), a cold current which diverts to the east from the EGC at about 72-73'N (Fig. 1). The Greenland Sea also represents the main route for water and heat exchange between the Arctic Ocean and the rest of the world. Ice is transported into the Greenland Sea from the Arctic Ocean and melts as it moves southward, so that the Greenland Sea as a whole, when averaged over a year, is an ice sink and thus a freshwater source. However, in winter local ice can also form within the Greenland Sea itself, associated with high ocean-atmosphere heat fluxes in the region influenced by the JMC. The consequent brine release acts locally to destabilise the surface layer and is one factor leading towards mid-gyre convection. The convection is associated with a carbon flux which is significant in the removal, or sequestration, of anthropogenic CO, from the atmosphere, since the surface waters in the region are found to be significantly undersaturated in dissolved CO,. The r6le of sea-ice is thus important on two scales. On the large scale, the sea-ice entering from the Arctic Basin via the Fram Strait contributes some 3000km3 per year to the freshwater budget of the Greenland Sea system, the largest single item in its overall net freshwater gain of some 1800km3 per year. On the mesoscale, the local ice growth in winter is a factor in the convection process. The area in

Seasonal Variation of Geostrophic Current and Water Transport in the Central Part of The Red Sea

The data from meteor R-V cruise legs-5 are used to discuss the circulation pattern and geostrophic current in the area extending between 19? and 24?N during February and July 1987. During winter season, the distributions of temperature, salinity, density and geostrophic current show the existence of gyratory pattern as a result of convection or advection processes. The extension of these gyres differs from surface to 250 m depth. About 7 gyres can be detected at the surface level and 4 gyres at 250 m depth. The existence of these gyres causes a strong cross currents. On the other hand the cyclonic gyratory pattern help in the renewal of deep Red Sea water. The pattern of flow along the east-west section consists of 2 layers. The surface one flowing to the north with net volume transport through this layer of about 1.975 × 106 m3/sec. While the lower flows to the south and with net volume transport equal to 0.984 × 106 m3/sec. During the summer season, the geostrophic currents appear in southeast direction in the area extending between 19 and 22?N, while at the rest of the investigated area (22-24?N) 6 gyres in between cyclonic and anticyclonic ones are present. At 250 m depth, the current problem is changed. In the area between 19-21?N, the currents flow in northwest direction and in the rest of the investigated area the currents were mainly in southwest direction accompanied with 4 different gyres. The water transport at the same east-west section is through two layers. The upper one moves to the south and the net volume transport is about 2.621 106 m3/sec. While the second layer was to the north with net volume transport of about 2.1368 × 106 m3/sec. استخدمت البيانات التي تم الحصول عليها من الرحلة 5 لسفينة الأبحاث الألمانية ميتيور في البحر الأحمر في مناقشة نظام 19 إلى 24رجة شمال. تمت القياسات البحرية خلال شهري فبراير ويوليو 1987 بما يتوافق مع تواد الرياح الموسمية الشمالية الشرقية والجنوبية الغربية العالية على التوالي. أيضا تم اختيار مستوى سطح البحر و250متر عمق كمستويين لتمثيل كل من الطبقة السطحية والطبقة المتوسطة حيث نوقشت الخواص الفيزيائية وحسبت السرعات الجيوستروفية لكل من الطبقة السطحية والطبقة المتوسطة. أظهرت توزيعات درجة الحرارة ، الملوحة والكثافة خلال فصل الشتاء وجود نظام دوامي يختلف في السطح عنه في الطبقة المتوسطة. كما أكد نظام التيارات وجود العديد من الدوامات (7دوامات على السطح و 4 على عمق 250متر). ويساعد وجود هذا النظام الدوراني على إعادة تكوين المياه العميقة في البحر الأحمر. ولقد وجد أن الحجم الانتقالي للمياه خلال قطاع ممتد من الشرق إلى الغرب لكل من الطبقة السطحية والمتوسطة حوالي 1,975×610 متر3 في اتجاه الشمال و0,984 × 610 متر3 في اتجاه الجنوب على التوالي. خلال فصل الصيف أظهرت حسابات التيارات الجيوستروفية وجود تيار جنوبي شرقي ممتد من 19 إلى 22 درجةشمال بينما توجد 6 دوامات سكلونية وانتيسكلونية في بقية منطقة الدراسة. انحرف التيار الجنوبي الشرقي إلى اتجاه جنوبي غربي خلال الطبقة المتوسطة في نفس المنطقة الممتدة من 19 إلى 21 درجة شمال بينما يستمر في اتجاه الجنوب الغربي في بقة منطقة الدراسة أيضا وجد إن حجم المياه المنقول على نفس القطاع الغربي بلغت حوالي 2,621 ×610، 2,137 ×610 متر3 لكل من الطبقة السطحية والطبقة المتوسطة.

The Hydrography of the Chupa Estuary, White Sea, Russia

This study was undertaken to determine the fate and fluxes of materials from Arctic estuaries to the coastal zone. The paper is the first of a series addressing questions relating to the physics and chemistry of the region. Three seasonal cruises were undertaken in the Chupa Estuary, White Sea, Russia; in summer (July 1994), autumn (September, 1995) and spring (May/June 1997). The Chupa is a fjord type estuary about 37km long with several deep troughs, connected by shallow sills, situated south of the Arctic circle on the western shore of Kandalaksha Bay. Vertical profiles were carried out on an axial grid of 10 stations, at spring and neap tidal states, during which measurements were made of salinity, temperature, current speed and direction, the concentration of suspended particulate material (SPM), pH, dissolved oxygen, nutrients (phosphate, silicate, nitrate, nitrite and ammonia) and particulate trace metals in sediments and SPM (Millwardet al., 1999, Estuarine, Coastal and Shelf Science,48,13–25). Additionally, sampling for the hydrodynamic determinants was carried out on diurnal anchor stations at key points in the estuary. Vertical stratification was pronounced, particularly in summer, with sharp gradients in temperature and salinity. A three layer vertical structure was observed with surface (0–5m) and deep (20–65m) water layers providing net down-estuary transport while the intermediate (5–20m) water layer drives net up-estuary transport and advection of more saline waters into the estuary. Strong internal waves were observed in the seaward half of the estuary at certain tidal states. The mechanism for renewal of deep waters in the troughs was investigated, this being considerably slower than in the near surface waters. Evidence of under-saturation of dissolved oxygen was evident in the deep waters of the troughs during the three surveys.

Sediments in Bottom-Arrested Gravity Plumes: Numerical Case Studies*

In the present paper a hydrostatic “reduced gravity” model, generally used to simulate transient bottom-arrested gravity plumes, was coupled with a sediment transport model. The coupled model considers the respective contribution of suspended sediment particles on the buoyancy of a plume and allows one to simulate autosuspension and size-differential deposition of sediments based on the local turbulence and settling velocities. Simulations using the coupled model reveal that sediment-enriched plumes are able to inject both entrained and original shelf water masses into intermediate and bottom layers of an adjacent ocean basin in an ageostrophic dynamical balance. Hence the mechanism described here is more rapid than classic, “seawater” plumes, which are solely driven by surplus density of the water masses. Results suggest that “turbidity” plumes may constitute an important process in the formation and renewal of deep waters in the Arctic Ocean. In case a turbidity plume reaches its level of equilibrium density, deposition of suspended particles causes the density of the interstitial fluid to be lower than the density of the ambient fluid. This initiates upward convection within the water column. The substantial difference between TS- and turbidity plumes is described by model experiments that utilize idealized slope and sediment distributions. A realistic simulation of a turbidity plume cascading down the continental slope of the western Barents Sea is presented. The computed distribution of deposited sediments agrees well with observations in an area of high accumulation of shelf-derived sediments. The frequency of occurrence of sediment-enriched gravity plumes originating from the Barents Sea shelf is estimated from the various geological variables (thickness of sediments at the bottom, grain size composition) measured from bottom sediments samples.

Distribution of helium and tritium in Lake Baikal

The 3H‐3He age of a water mass is a measure of the time that has passed since the water mass was last in contact with the atmosphere. Between 1992 and 1995 a detailed study of 3H‐3He ages was conducted in Lake Baikal, the deepest and largest lake by volume on Earth, to investigate deep water renewal in its three major basins. Maximum 3H‐3He ages are 14–17 years in the southern basin, 16–18 years in the central basin, and 10–11 years in the northern basin. Rates of renewal of deep water with surface water, deduced from volume‐weighted mean 3H‐3He ages below 250 m depth, are about 10% yr−1 in the southern and central basins and 15% yr−1 in the northern basin. In the southern basin the mean 3H‐3He age below 250 m depth increased steadily from 9.6 years in 1992 to 11 years in 1995, indicating a slight diminution in deep water renewal during this time. Bottom water renewal by large‐scale advection was estimated from the mass balance of 3He in the 200 m thick bottom layer of each basin. Bottom water renewal rates in the northern basin were found to be between 80 and 150 km3 yr−1 and in the central basin between 10 and 20 km3 yr−1, whereas in the southern basin they were practically zero. Correlating oxygen and dissolved helium‐4 concentrations with 3H‐3He age allowed us to determine the mean hypolimnetic oxygen depletion rate in the water column (4.5 μmol L−1 yr−1), as well as mean helium fluxes from the lake bottom (2.8×1011 atoms m−2 s−1 in the northern basin, and 1.3×1011 atoms m−2 s−1 in the central and southern basins). The helium isotope ratio of the terrigenic helium component injected from the lake bottom, determined from measurements of water from hydrothermal springs in the vicinity of the lake, was found to be ∼2.2×10−7.

Redox- and Mixing-Dependent Phosphorus Fluxes in Permanently Density Stratified Anoxic Natural Water

Lake Lugano (northern subbasin) is a permanently density stratified perialpine lake in the southem Swiss Alps. Wind protection due to the mountainous environment and the great depth of about 300m are the reasons for reduced mixing, low deep water renewal rates and anoxic conditions below 80 m depth. Consequently, during the last 20 years a slow accumulation of salts and nutrients has occurred in the deep water and in the sediments. Steady-state physical and geochemical conditions allow to study diffusive fluxes and particular phosphorus cycling responding to changes of the external forcing occurring on medium time scales (15 years). Measurements of nutrients in the sediments and the water colunm have been carried out in order to determine rates of diffusive fluxes. Using a SeaBird CTD profiler, temperature, conductivity and oxygen were measured to quantify mixing rates.

The Roles of Hydrographic and Biogeochemical Processes in the Distribution of Dissolved Inorganic Nutrients in a Scottish Sea-loch: Consequences for the Spring Phytoplankton Bloom

Observations taken in Loch Linnhe, a fjord on the west coast of Scotland in 1992, indicate that nutrient distributions are controlled by physical mixing processes (conservative behaviour), temporally varying end-member concentrations (apparently non-conservative behaviour) and biogeochemical processes (real non-conservative behaviour). One-dimensional steady-state mixing diagrams conventionally employed for estuarine nutrient studies cannot be used for Loch Linnhe because of the complex hydrography observed. A one-dimensional box model has been developed which accurately reproduces the hydrography of the system and has been used to isolate and quantify the different types of nutrient behaviour in the system: non-conservative behaviour is identified in the isolated bottom-waters of the basin and apparently non-conservative behaviour is identified at depths greater than 60m. Chlorophyll data show the onset of a phytoplankton bloom coinciding with a deep water renewal event. A similar sequence of events was also noted in 1990 and 1991 and it is suggested that these events are linked.